1. Field of the Invention
The present invention relates to interferometric optical sensor systems employing active coherence reduction of the source light.
2. Description of the Related Art
The coherence function of an optical signal versus delay τ and time t is defined as the autocorrelation function of the normalized field phasor E(t) of the optical signal. In other words, the coherence function R(τ,t) equals the autocorrelation function of the light and is given by
                              R          ⁡                      (                          τ              ,              t                        )                          =                              ∫                          t              -                              T                2                                                    t              +                              T                2                                              ⁢                                                    E                *                            ⁡                              (                                  t                  ′                                )                                      ⁢                          E              ⁡                              (                                                      t                    ′                                    +                  τ                                )                                      ⁢                          w              ⁡                              (                                                      t                    ′                                    -                  t                                )                                      ⁢                          ⅆ                              t                ′                                                                        (        1        )            In the common mathematical definition of the coherence function the integration time T in equation (1) approaches infinity, while w(t) is independent on t and equals 1/T. If two optical field phasors E(t) and E(t+τ) originating from the same source with delay difference τ are combined on a detector, the visibility of the interference signal output from the detector will be proportional to the magnitude of an effective coherence function R(τ,t), which is still given by equation (1), but where w(t) equals the impulse response of the detector. If w(t) also includes the effect of electrical or digital receiver filters attached to the detector output, R(τ,t) describes the visibility of the output signals from the receiver filters. In equation (1), w(t) represents a moving average weighting function that is multiplied with the interference power term E19(t′)E(t′+τ). Normalization of the field phasor E(t) means that the field phasor is scaled such that R(0,t)=1 on the average.
It can be shown that the coherence function of the light can be defined as the Fourier transform of its optical power spectrum. The coherence time may be defined as the full width at half maximum (FWHM) of the autocorrelation function, and it can be shown that the coherence time is inversely proportional to the bandwidth of the optical power spectrum. The term “coherence length” is used for the distance that the light will travel within the coherence time. The effective coherence function discussed above can be defined as the Fourier transform of the optical power spectrum after convolution with the Fourier transform of w(t). This corresponds to taking the Fourier transform of the optical power spectrum measured with resolution bandwidth that corresponds to the bandwidth of w(t), i.e. the detector bandwidth. The effective coherence time is then the full width at half maximum (FWHM) of the effective autocorrelation function, and the effective coherence length is the distance that the light will travel within the effective coherence time.
In most practical interferometric applications it is the effective coherence function, where w(t) equals the impulse response of the detector including filters, that is of interest, and in the following we use the terms coherence function, coherence time, and coherence length when we mean the effective coherence function, effective coherence time, effective and coherence length.
Interferometric optical sensor systems will typically comprise an optical source unit, which produces an optical signal. If wavelength division multiplexing of sensor interferometers is employed, this signal may typically comprise a multiple of optical signals, each signal being confined to a separate wavelength range defining a wavelength channel. Such a multiwavelength channel source may typically comprise a multiple of laser sources operating in different wavelength channels, and a wavelength division multiplexer arranged to combine the different wavelength signals. If time division multiplexing of sensor interferometers is employed, the optical signal from the source unit may typically comprise pulses.
The optical signal from the source unit is launched into an optical network comprising a multiple of optical pathways from its input to its output, and where some pairs of optical pathways form sensor interferometers. The difference in delay between two paths forming a sensor interferometer is called the sensor delay or imbalance of that sensor. The optical network may typically use optical waveguides such as optical fiber for guiding of the optical signals. If wavelength division multiplexing is employed the optical network may typically comprise wavelength dependent couplers or wavelength dependent reflectors such as fiber Bragg grating (FBG) reflectors, arranged in a manner such that optical signals belonging to a wavelength channel will only propagate through a limited set of the paths through the network. Thus, different sensors can be interrogated with light in different wavelength channels.
Light emerging from the output of the optical network is typically directed to a detection unit. If wavelength division multiplexing is employed the detection unit may typically comprise a wavelength division demultiplexer which separates the different wavelength channel components of the incoming light and directs the separated components to corresponding wavelength channel detectors. The detectors will typically convert the incoming light signals to output voltage or current signals that are proportional to the optical power.
The electrical signals emerging from the signal processing unit will typically be analyzed by some signal processing means to extract information dependent on the phase of the sensor interferometers, defined as the difference in phase delay experienced by the interrogating optical signal when traveling in the two arms of a sensor interferometer. The phase of a sensor interferometer is linearly dependent on the exact sensor delay of the interferometer. This information may typically carry useful information about physical parameters acting differently on the two pathways comprising each sensor interferometer. Examples of such physical parameters are acoustic vibrations or pressure fluctuations, temperature, or hydrostatic pressure. Some sensor interferometers may also be designed to be insensitive to physical parameters that one wants to measure, and rather be used as reference sensors to correct the readout from other sensor interferometers for influences from physical parameter fluctuations that one does not want to measure, but which affect the measurements from both the reference sensor and the corrected sensor. The signal processing means may typically comprise components such as analog mixers, sample and hold circuits, analog to digital converters, microprocessors, digital signal processors, etc.
The sensor system may also comprise a compensating interferometer. A compensating interferometer comprises two optical paths from its input to its output with a path imbalance, i.e. difference in transmission delay between the two paths, that is chosen to be approximately equal to that of the path delay of the sensor interferometer. The compensating interferometer is connected in series with the sensor interferometer, either after the source unit at the optical transmitter end (in which case it is called a transmitter interferometer in parts of the existing literature) or before the detector unit at the receiver end (in which case it is called a receiver interferometer in parts of the existing literature).
The compensating interferometer ensures that there will be for each sensor interferometer a pair of pathways from the source unit to the detection unit going through both the compensating interferometer and the optical network (with the compensating interferometer placed either before or after the optical network) that has a delay imbalance that is close to zero, i.e. much shorter than the sensor interferometer delay. Since the sensitivity of the interference phase to source frequency fluctuations is proportional to the delay imbalance of the optical pathways that the interfering waves have traveled, the use of a compensating interferometer can allow for the use of cheaper light sources with a lower optical frequency stability or phase stability and lower coherence, as opposed to systems that do not employ compensating interferometers. The level of frequency fluctuations that can be allowed is decided by the production uncertainty or spread in the mismatch between the compensating interferometer delay difference and the sensor interferometer delay differences. For fiber optic interferometric sensor systems this spread can depend on uncertainties in the fiber splicing process and fiber strain levels, as well as in some cases the flexibility of placement of fiber splices within the sensor housing. The uncertainty can typically be in the range of 1 to 50 mm in fiber length, corresponding to delay variations in the order of 0.01 to 0.5 ns for a dual path fiber in a reflector-based interferometer. In sensor systems comprising compensating interferometers, pairs of pathways with delay imbalances close to one and two times the sensor interferometer delay will also exist. Interference between light components with such delay imbalances can lead to nonlinear responses and noise in the sensor readout. In pulsed multiplexed systems, these interference terms are removed by pulsing of the source with pulses that are shorter than the sensor interferometer delay, resulting in that the wanted interference between pulse components that have experienced approximately equal delays from the source unit to the detection unit will be separated in time from pulse components that have experienced unequal delays. The wanted interference signal can thus be separated and extracted by time gating or discrete time sampling of the output signals from the detection unit. Due to the pulsed nature of the interrogation signals such systems can readily be adapted for time division multiplexing (TDM). Sensors belonging to different TDM channels will then have different offset transmission delays from the source unit to the detection unit, so that detected interference signals from the different sensors can be separated in the time domain by time gating or discrete time sampling of the output signals from the detection unit.
Various approaches have been disclosed for extracting the sensor phase. Most of them rely on varying the interference phase of the sensor interferometers actively as a function of time through modulation of the phase or frequency of the interrogating optical signal or by modulation of the interferometer imbalance. This ensures that the signal processing means can extract both in-phase and quadrature information about the interference of each sensor interferometer by analyzing the output signals from the detection unit as a function of time, thus enabling the interference phase to be extracted without sign ambiguity. One may for example employ the “phase generated carrier” (PGC) demodulation approach disclosed in the Homodyne Demodulation Scheme for Fiber Optic Sensors Using Phase Generated Carrier by A. B. A. Dandridge et al. published in IEEE J. of Quantum Electronics, Vol. QE-18, pp. 1647-1653, 1982, wherein the term PGC refers to the carrier frequencies generated at the detector at the frequency at which the interference phase is actively modulated and at harmonics of this frequency. The sensor interferometer phase can be extracted without sign ambiguity by analyzing the detector signals in a frequency band comprising minimum two of the generated frequencies. The interference phase modulation can be generated in several ways, for instance by modulation of the optical source frequency, modulation of the optical phase or frequency outside the source, or by modulating the delay in one of the interferometer arms. If a compensating interferometer is employed, interference phase modulation can be generated by modulation of the phase delay in one of the arms of the compensating interferometer. Systems where the optical signal component traveling in one of the pathways of a sensor interferometer is frequency shifted relatively to the optical signal component traveling in the other pathway of the same sensor interferometer may also be used to generate a heterodyne signal at the detector, as described in U.S. Pat. No. 6,466,706 entitled “Pulsed System and Method for Fiber Optic Sensor,” resulting in a carrier signal at the detector onto which the sensor interferometer phase is encoded and can be extracted without sign ambiguity. For most of the demodulation approaches based on the PGC or heterodyning techniques, PGC frequencies or optical frequency shifts, respectively, that are at least larger than two times the readout frequency bandwidth of the demodulated sensor phase signal are required to avoid frequency overlap of the detected carrier sidebands and to avoid nonlinearities and errors in the demodulated output signals.
Phase demodulation without sign ambiguity can also be achieved without any modulation of the interference phase or generation of carrier frequencies at the detectors. For instance, a compensating interferometer placed in front of the detection unit with outputs from a 3×3 fiber coupler to two or three detectors may be used, as disclosed for a pulsed system in U.S. Pat. No. 5,946,429 entitled “Time-Division Multiplexing of Polarization-insensitive Fiber Optic Michelson Interferometric Sensor.” The interference signals at the outputs from the 3×3 coupler will then be phase shifted relative to each other, thus providing both in-phase and quadrature information about the interference signal to the signal processing means.
The detection unit has a detector bandwidth that is capable of capturing all the information required by the signal and processing unit to demodulate the sensor interferometer phase with the required demodulated phase signal bandwidth. With PGC demodulation techniques the necessary detection bandwidth may typically include from 2 to 12 harmonics of the PGC frequency. With heterodyne demodulation techniques the necessary detection bandwidth may typically be in the order of one to two times the heterodyne frequency shift. With demodulation techniques employing a 3×3 fiber coupler in front of the detection unit, the necessary detection bandwidth may typically be in the order of one to a few times the required demodulated phase signal bandwidth. Due to nonlinearities in the interference phase to fringe signal response, even higher detection bandwidths may be required if the demodulated phase signal amplitude is high.
In systems employing a pulsed optical source the necessary detection bandwidth must be sufficient to avoid unwanted crosstalk in the time domain between subsequent pulses, and the necessary detection bandwidth will typically be in the order of the inverse of the pulse duration, i.e. the inverse of the sensor interferometer delay.
Additional components may also be included in the interferometric sensor system, such as for example optical amplifiers to boost the optical power emerging from the source unit before it is launched into the system, polarization controllers, power supplies, optical circulators, optical modulators for modulating the sensor interferometer phase, and more.
Interferometric sensor systems employing pulsed sources with a coherence length that is even shorter than the pulse length in combination with compensating interferometers are known from the prior art. Due to the pulsed nature of the interrogation signals such systems can readily be adapted for time division multiplexing. Pulses with duration shorter than the interferometer imbalance are generated by the source. The fraction of a pulse that follows the short path through the sensor interferometer and the long path through the compensating interferometer will then overlap at the detector with the fraction of the same pulse that follows the long path through the sensor interferometer and the short path through the compensating interferometer.
In most of the prior art references employing short coherence sources, a coherence time that is shorter than the pulse length is achieved through inherent random processes in the source such as spontaneous emission or thermal radiation. However, such random processes correspond to random fluctuations in the source frequency or phase. If the compensating interferometer delay is not perfectly matched to the sensor interferometer delay, these random frequency fluctuations will cause unwanted noise fluctuations in the readout phase, as discussed above. The '706 patent discloses an alternative approach where the optical field phasor (i.e. the complex field amplitude) of the light emerging from a coherent source is modulated in a controlled and repetitive manner by chirping the optical frequency within each pulse delivered by the source unit with an acoustooptic modulator. This ensures that the mean optical frequency of the source is not disturbed from pulse to pulse, and thus conversion from source frequency fluctuations to noise in the demodulated sensor phase signal is avoided. The minimum coherence time that can be achieved by coherence modulation using this technique is limited by the response time or the duration of the impulse response of the modulator, which is fundamentally limited by the speed of sound in the acoustooptic interaction medium to the range from 5 to 100 ns for high speed modulators, and the price and complexity of the modulators increases with increasing speed. This imposes a limit to how much the coherence time can be reduced by this technique, and thus a limit to how much the unwanted effects of the source coherence, which are discussed below, can be suppressed.
The use of low coherent sources provides several advantages, including reduced noise, crosstalk and harmonic distortion in the sensor response from interference with unwanted reflections such as Rayleigh scattering, reflections from other sensors multiplexed on the same fiber, connectors, etc. Essentially, only reflectors that are separated from the sensor reflectors by less than the coherence length of the source will contribute to errors in the demodulated signal.
If the lead fiber is of substantial length, distributed Rayleigh scattering may cause a significant amount of noise at the detectors and thus in the demodulated sensor interferometer phase signals. It can be shown that the squared Rayleigh noise contribution to the detector signal output is proportional to,
                              1                      2            ⁢                          T              f                                      ⁢                              ∫                          -                              T                f                                                    T              f                                ⁢                                                                                      R                  ⁡                                      (                                          τ                      ,                      t                                        )                                                                              2                        ⁢                          ⅆ              τ                                                          (        2        )            where Tf is the transmission delay through the fiber contributing with Rayleigh noise to the demodulated phase signal. It is thus desirable to get the integral expression in (2), which represents a Rayleigh noise suppression factor, as small as possible. If the coherence function has only one peak versus τ, the integral will be directly proportional to the coherence time.
In systems employing pulsed interrogation, reflections with delay spacing from the interferometer that equals a multiple of the interrogation pulse period will interfere with the sensor reflections. If subsequent pulses are correlated with a stable or slowly varying phase relation, such reflections will contribute to crosstalk and harmonic distortion. If subsequent pulses are not correlated and the pulse phase relation varies in a random fashion such reflections will contribute to noise in the demodulated phase signal. In systems employing a common down lead and up lead fiber any losses in the lead fiber, due to for instance connector losses or directional couplers, will reduce the ratio of the reflected signal pulse amplitudes from the sensor interferometers to unwanted reflections from higher up in the lead fiber. Interference with unwanted reflections can therefore significantly degrade the quality of the demodulated readout signal.
As already mentioned, the combination of a compensating interferometer and a low coherence source reduces the requirements on the source frequency stability, since the readout phase is proportional to the product of the optical source frequency and the delay imbalance of the interfering pulses. For example, in a system with a sensor interferometer delay of τ=100 ns (corresponding to 20 m of single pass delay in optical fiber), a readout phase resolution of Δφ=1 mrad requires that the source frequency has fluctuations less than Δφ/(2πτ)=160 Hz within the demodulated bandwidth of interest if a compensating interferometer is not used. This requires advanced and expensive laser sources that must be isolated from vibrations. When a compensating interferometer that matches the sensor interferometer within Δτ=0.1 ns (20 mm fiber) is used, the source frequency stability requirements are relaxed by three orders of magnitude to Δφ/(2πΔτ)=160 kHz. Due to uncertainties in fiber strain and in the fiber splicing process involved it is hard to achieve delay matching better than the order of 0.01 to 0.5 ns.
The use of low coherence sources also increases the threshold for unwanted Brillouin scattering in systems employing long lead fibers to reach remote sensor locations. The optical input power required to overcome shot noise limitations of the detector (receiver) can be high, especially if optical losses are high. In such cases, the input optical power required to overcome shot noise may exceed the threshold for stimulated Brillouin scattering (SBS) if a highly coherent source is launched into a long lead fiber. If the SBS threshold is exceeded, a large fraction of the optical input signal is scattered by phonons, which are generated due to the high optical power. This causes a large reduction of the optical power reaching the sensor (effective loss). If a common optical fiber is used for transmission to and from the sensor (as in reflective sensor systems) SBS will lead to a large signal superimposed on the reflected sensor response. Instabilities in the SBS process may also cause severe noise in the readout signal. Acceptable system performance can therefore not be achieved when the SBS threshold is exceeded.
Provided that the fiber transmission loss is less than a few dB, in a monochromatic optical source, the SBS threshold power is inversely proportional to the lead fiber length. For higher losses (assuming a given attenuation per km and increasing fiber length), the threshold power approaches a constant level. If the bandwidth BW of the source exceeds the gain bandwidth of the SBS process, which for silica fiber may be in the range of BWSBS=20 to 100 MHz, then the SBS threshold will also be proportional to the bandwidth ratio BW/BWSBS, where BW is the optical bandwidth of the source. More precisely, the threshold condition is determined by the peak of the optical power spectral density of the source averaged with an optical resolution bandwidth of BWSBS. BWSBS depends on the lifetime of the stimulated phonons in the fiber.
Some prior art references exist where the coherence function of the source is synthesized to have a peak at a chosen delay by modulating the source field phasor in a periodic manner, either by modulating the drive current of a source laser or by use of an external modulator. Peaks in the coherence function will then occur at multiples of the modulation period. In U.S. Pat. No. 4,818,064 entitled “Sensor Array and Method of Selective Interferometric Sensing by Use of Coherence Synthesis,” this technique is used to select to which interferometer among a multiple of sensor interferometers with different sensor delay imbalances that the demodulation should be sensitive. By varying the modulation period sensors with different delay imbalances can be selected. This type of coherence synthesis provides some of the same advantages with respect to suppression unwanted effects of Rayleigh and other spurious reflections as well as stimulated Brillouin scattering as other techniques employing low coherence sources for interferometric sensor interrogation. However, since the coherence function becomes a periodic function of delay with a repetition period equal to the sensor interferometer delay, the readout will be sensitive to Rayleigh and spurious reflections that introduces pathways from the source unit to the detection unit that is spaced by any multiple of the sensor interferometer delay from the transmission delays of the sensor interferometer paths. In other words, the Rayleigh noise suppression factor as defined in equation (2) will contain unwanted contributions from a large number of coherence peaks. For comparison, the pulsed source unit described in the '706 patent will have a coherence function with peaks that repeat for every pulse repetition interval, which is typically much longer than the sensor delay. Another shortcoming of the technique disclosed in the '064 patent is that the sensitivity to fluctuations in the mean source frequency (i.e. laser frequency fluctuations) is much higher than for systems employing a compensating interferometer.
In general, interferometer interrogation techniques employing continuous wave and pulsed sources have different advantages and disadvantages that make them preferable for different applications. Pulsed source interrogation allows for time division multiplexing in addition to wavelength division multiplexing, and may therefore be advantageous for systems where multiplexing of a large number of interferometers is required. On the other hand the short dutycycle of the detected interference pulses means that rather high optical pulse powers are required to overcome the fundamental shot noise limitation of optical detection. This can be overcome by increasing the source power, for instance by incorporating a relatively expensive optical amplifier. However, in sensor systems with long transmission lead fibers to the sensor location and in addition possible significant transmission losses near the sensors the power requirement may become so high that nonlinear processes like self phase modulation and Raman power transfer, cross phase modulation, or four wave mixing and between wavelength channels may lead to problems by introducing excess noise and effective loss mechanisms to the transmitted optical signal. Furthermore, the pulsed approach requires very high speed components which may be relatively expensive such as high speed intensity modulation means for switching and high speed detection and sampling electronics. For low cost systems that do not require time division multiplexing of too many sensors and for systems where transmission losses and lead fiber lengths are large, continuous wave systems may thus provide an advantage over pulsed systems.
Thus, there exists a need for improved techniques for interrogation in interferometric sensor systems employing pulsed sources that reduce readout interferometer phase errors to Rayleigh scattering, spurious reflections, or stimulated Brillouin scattering, and which overcomes other problems with the prior art mentioned herein. There exists a further need for improved techniques for interrogation in interferometric sensor systems employing continuous wave sources that reduce readout interferometer phase errors caused by Rayleigh scattering, spurious reflections, or stimulated Brillouin scattering, and which overcomes other problems with the prior art mentioned herein.